In recent years, along with the developments of multifunctional portable communication terminals with multiple bands, the number of parts constituting a so-called RF front-end portion also increases; however, since there have been demands for a small size and light weight in the portable communication terminal in its own structure, there have been further demands for a small size and light weight in the respective parts in comparison with conventional parts.
Among the respective parts of the portable communication terminal, there is a duplexer which is used right below the antenna. The duplexer refers to a device having a function for separating signals having different frequency bands, and, for example, a duplexer for separating a transmission frequency band and a receiving frequency band from each other is used in a portable communication terminal or the like of a CDMA system.
Conventionally, a duplexer which uses a dielectric resonator filter has been used; however, in recent years, because of demands for small sizes, a duplexer which uses an elastic surface wave filter has been used. The elastic surface wave filter is an element which uses elastic surface waves that are excited by an IDT (Inter Digital Transducer) placed on a piezoelectric substrate (for example, see JP-A No. 05-167388).
The duplexer is constructed by connecting a plurality of filter devices having different frequency pass bands with one another. FIG. 2 is a drawing that schematically shows a circuit structure of a duplexer D1 as a general example. In the duplexer D1, a first filter F1 and a second filter F2 are connected by a common electrode 4000, and an antenna terminal 1000 is connected to the common electrode 4000. For example, when the first filter F1 serves as a filter (hereinafter, referred to as Tx filter) that allows the transmission frequency band to pass while the second filter F2 serves as a filter (hereinafter, referred to as Rx filter) that allows the receiving frequency band to pass, a signal received by the antenna terminal 1000 is sent to a receiving circuit, not shown, from an output terminal 2000 through the second filter F2 (Rx filter), and a signal from the transmission circuit, not shown, is sent from the input terminal 2000 through the first filter F1 (Tx filter) to the antenna terminal 1000 from which it is transmitted.
In a circuit structure shown in FIG. 2, however, for example, the transmission signal is not only sent from the common electrode 4000 to the antenna terminal 1000, but also leaked toward the receiving circuit side. Therefore, a matching circuit L1 is disposed between the antenna terminal 1000 and the respective filters F1, F2. The matching circuit L1 functions so that the transmission circuit is allowed to have a virtually infinite impedance in the receiving frequency band for the antenna terminal 1000, and so that the receiving circuit is allowed to have a virtually infinite impedance in the transmitting frequency band for the transmission circuit. Here, the numeric value indicating a degree of a signal leaking from the transmission circuit to the receiving circuit is generally referred to as an isolation characteristic, which indicates that the greater the value, the smaller the leakage.
The duplexer is required to have a high isolation characteristic in the transmission band as well as in the receiving band. In the conventional duplexer, in order to achieve a small size, the respective resonators are normally placed closely to each other. However, in such a structure, a signal is leaked between the Tx filter and the Rx filter, failing to avoid degradation in the isolation characteristic.
With respect to this problem, JP-A No. 2004-48240 has disclosed a structure in which, in a DMS-type filter, a plurality of pairs of comb-shaped electrodes are disposed in such a manner that main propagating directions of the respective elastic waves are not overlapped with each other; thus, the Patent Document describes that it becomes possible to avoid propagating waves between the electrodes from interfering with each other, and consequently to reduce spurious characteristics (for example, see claim 11, FIG. 2, etc.).
Since the DMS-type filter generally has few resonators, it is comparatively easy to dispose the comb-shaped electrodes so that main propagating directions of the respective elastic waves are not overlapped with each other, as described above. However, in the case of the duplexer that requires a power resistant property, a ladder-type filter rather than a DMS-type filter tends to be used. Here, the ladder-type filter has many resonators; therefore, when an attempt is made to apply the technique disclosed by JP-A No. 2004-48240 to the ladder-type filter so that the comb-shaped electrodes are mutually disposed, with propagating paths of the respective elastic waves being deviated from each other, a problem arises in which it becomes difficult to achieve a small size.